Conventionally, many pierce nuts are used in vehicles, typically automobiles. In general, a pierce nut includes a female screw thread portion formed in the center of a nut body, an annular piercing portion projecting around the female screw thread portion, and an annular outer peripheral projection extending from the periphery of the nut body.
Therefore, by driving the pierce nut into a steel plate, which is a mating member of the nut, the piercing portion runs through the mating member to make a circular hole therein, and at the same time, plastically deforms the steel plate around the peripheral edge portion of the hole by virtue of plasticity such that the steel plate flows into an annular groove formed between the piercing portion and the outer peripheral projection. Consequently, the pierce nut is securely fixed to the mating member. Thus, the pierce nut is used for screw-attaching another component.
Compared to a weld nut, which is welded and fixed to a mating member, a pierce nut has an advantage not only in that the need to form a prepared hole in the mating member is eliminated, but also in that the need to partially heat the mating member is eliminated. Accordingly, many pierce nuts are used as vehicle components in particular.
However, in recent years, use of a high-strength steel plate called “high-tension steel plate” has been increasing in order to reduce the weight of an automobile. This high-strength steel plate is harder and less likely to be deformed than an ordinary steel plate. Therefore, a conventional pierce nut has a problem in that its peeling strength after attachment insufficient.
FIGS. 1(A) and 1(B) are sectional views illustrating the foregoing state. FIG. 1(A) shows the case of an ordinary steel plate, and FIG. 1(B) shows the case of a high-strength steel plate. In FIG. 1(A), the steel plate S plastically flows into an annular groove 3 between the piercing portion 1 and the outer peripheral projection 2 such that the pierce nut is securely fixed to the steel plate S. The inner peripheral surface of the outer peripheral projection 2 forms a sloping surface decreasing in diameter toward the leading end thereof. In this description, the leading end refers to the direction in which the end is further away from the nut body in the axial direction thereof.
However, in FIG. 1(B), since the hardness of the steel plate S is high, the steel plate S cannot sufficiently flow into the annular groove 3. Moreover, the outer peripheral projection 2 of the pierce nut is liable to be deformed such that this projection 2 inclines inward. Consequently, the strength of the pierce nut against separation from the steel plate S decreases, and also contact of the steel plate S with the outer peripheral projection 2 is insufficient, resulting in insufficient rotation torque.
A pierce nut for a high-strength steel plate is disclosed in Patent Document 1. The pierce nut for a high-strength steel plate does not include a conventional outer peripheral projection 2. However, in place of the projection 2, this pierce nut has an outer groove, an annular projection, and an inner groove in order to enhance the contact between the high-strength steel plate and the pierce nut, thereby ensuring rotation torque. However, complete elimination of the outer peripheral projection 2 may lead to a decrease in peeling strength of the pierce nut from the steel plate.